Field of the Disclosure
Embodiments of the present application relate to the art of a vehicle control system configured to control an internal combustion engine that is operated with different air/fuel ratios and an automatic transmission that changes an input speed from the engine.
Discussion of the Related Art
In the conventional lean-burn engines, air/fuel ratio can be altered between stoichiometric ratio in which the engine is run at the stoichiometric point and lean-burn ratio in which a ratio of fuel to air is decreased to save the fuel.
Whereas, in the automatic transmission, a shift shock may be caused by pulsation of an input torque from the engine. In the conventional vehicle, in order to suppress such shift shock by reducing an input torque to the transmission, an ignition timing of the engine is retarded from the so-called MBT (Minimum advance for the Best Torque) timing at which the maximum cylinder pressure will occur.
JP-A-11-170892 describes a control device for reducing a shift shock. According to the teachings of JP-A-11-170892, the control device is configured to inhibit a switching control of air/fuel ratio between the lean-burn ratio and the stoichiometric ratio during a shifting operation of gear stage of a transmission, and to inhibit the shifting operation of the transmission during execution of the switching control of air/fuel ratio.
JP-A-05-104990 describes a shock reducing device for powertrain configured to reduce shock of the powertrain by carrying out a switching control of engine output characteristic simultaneously with a shift control of gear stage of a transmission.
JP-A-2003-214216 also describes a control device and a control method of the lean-burn engine. According to the teachings of JP-A-2003-214216, a switching demand of air/fuel ratio from the stoichiometric ratio to the lean-burn ratio is limited if an engine load is fluctuated by an accelerating operation, a clutch operation, or a speed change operation.
However, a shifting operation of the transmission may be executed before switching the air/fuel ratio if required before the switching operation of air/fuel ratio is required. In this case, if the engine is operated in the lean-burn mode in which an ignition timing of the engine is retarded almost to the maximum retarded timing, an engine misfire may be caused by further retarding the ignition timing. Consequently, shift shock may be caused during shifting operation of the transmission.
In order to avoid such misfire of the engine, it is required to develop a technique for promptly carrying out a torque-down control such as the ignition retard during shifting operation of the transmission, while switching air/fuel ratio from the lean-burn ratio to the stoichiometric ratio.